1. Field of the Invention
The present invention relates to a power amplifier for mobile communications such as mobile telephones.
2. Background Art
Recently, as power amplifiers for mobile telephones represented by CDMA (Code Division Multiple Access), a GaAs-HBT (Hetero-junction Bipolar Transistor) power amplifier has been broadly used (for example, refer to Japanese Patent Application Laid-Open No. 2004-343244). These power amplifiers are mainly operated in middle and low output in urban areas where base stations are relatively concentrated. For this reason, the improvement of the operation efficiency in the times of middle and low output operations is effective for the elongation of the talk time of mobile telephone. Therefore, the improvement of operation efficiency in the time of middle and low output operations (ex. 0 to 17 dBm) in addition to the time of high output operations (ex. 28 dBm) has become important. To improve the operation efficiency in the time of middle and low output operations, a method to lower the collector voltage of an amplifier using a DC/DC convertor according to output power has been known.
FIG. 9 is a graph showing the relationship between the output power and distortion of the amplifier. FIG. 10 is a graph showing the relationship between the output power and the operation efficiency of the amplifier. If the collector voltage is lowered (from Vc_High to Vc_Mid), then the efficiency is improved, but the distortion characteristics are deteriorated. In addition, although the distortion characteristics are improved as the output power is decreased, the operation efficiency is decreased. Therefore, by lowering the collector voltage, and by inhibiting the output power in the range to satisfy the standards of distortion characteristics, the operation efficiency is improved.